Nomenclature & Conformations of Alkanes & Cycloalkanes Chapter 4 Nomenclature & Conformations of Alkanes & Cycloalkanes

About The Authors These Powerpoint Lecture Slides were created and prepared by Professor William Tam and his wife Dr. Phillis Chang. Professor William Tam received his B.Sc. at the University of Hong Kong in 1990 and his Ph.D. at the University of Toronto (Canada) in 1995. He was an NSERC postdoctoral fellow at the Imperial College (UK) and at Harvard University (USA). He joined the Department of Chemistry at the University of Guelph (Ontario, Canada) in 1998 and is currently a Full Professor and Associate Chair in the department. Professor Tam has received several awards in research and teaching, and according to Essential Science Indicators, he is currently ranked as the Top 1% most cited Chemists worldwide. He has published four books and over 80 scientific papers in top international journals such as J. Am. Chem. Soc., Angew. Chem., Org. Lett., and J. Org. Chem. Dr. Phillis Chang received her B.Sc. at New York University (USA) in 1994, her M.Sc. and Ph.D. in 1997 and 2001 at the University of Guelph (Canada). She lives in Guelph with her husband, William, and their son, Matthew.

Introduction to Alkanes & Cycloalkanes Alkanes and cycloalkanes are hydrocarbons in which all the carbon-carbon (C–C) bonds are single bonds Hydrocarbons that contain C═C: Alkenes Hydrocarbons that contain C≡C: Alkynes

Alkanes: CnH2n+2 Cycloalkanes: CnH2n

1A. Sources of Alkanes: Petroleum Petroleum is the primary source of alkanes. It is a complex mixture of mostly alkanes and aromatic hydrocarbons with small amounts of oxygen-, nitrogen-, and sulfur-containing compounds

Petroleum refining Distillation is the first step in refining petroleum. Its components are separated based on different volatility More than 500 different compounds are contained in petroleum distillates boiling below 200oC

Petroleum refining (Cont’d) The fractions taken contain a mixture of alkanes of similar boiling points Mixture of alkanes can be used as fuels, solvents, and lubricants

Gasoline The demand of gasoline is much greater than that supplied by the gasoline fraction of petroleum Converting hydrocarbons from other fractions of petroleum into gasoline by “catalytic cracking”

Gasoline (Cont’d) Isooctane burns very smoothly (without knocking) in internal combustion engines and is used as one of the standards by which the octane rating of gasoline is established

Gasoline (Cont’d) e.g. a gasoline of a mixture: 87% isooctane and 13% heptane Rated as 87-octane gasoline

Typical Fractions Obtained by Distillation of Petroleum Boiling Range of Fraction (oC) # of Carbon Atoms per Molecule Use Below 20 C1 – C4 Natural gas, bottled gas, petrochemicals 20 – 60 C5 – C6 Petroleum ether, solvents 60 – 100 C6 – C7 Ligroin, solvents 40 – 200 C5 – C10 Gasoline (straight-run gasoline) 175 – 325 C12 – C18 Kerosene and jet fuel

Typical Fractions Obtained by Distillation of Petroleum (Cont’d) Boiling Range of Fraction (oC) # of Carbon Atoms per Molecule Use 250 – 400 C12 and higher Gas oil, fuel oil, and diesel oil Nonvolatile liquids C20 and higher Refined mineral oil, lubricating oil, and grease Nonvolatile solids Paraffin wax, asphalt, and tar

Shapes of Alkanes All carbon atoms in alkanes and cycloalkanes are sp3 hybridized, and they all have a tetrahedral geometry Even “straight-chain” alkanes are not straight. They have a zigzag geometry

“Straight-chain” (unbranched) alkanes

Branched-chain alkanes

Butane and isobutane have the same molecular formula (C4H10) but different bond connectivities. Such compounds are called constitutional isomers

C4 and higher alkanes exist as constitutional isomers C4 and higher alkanes exist as constitutional isomers. The number of constitutional isomers increases rapidly with the carbon number Molecular Formula # of Possible Const. Isomers C4H10 2 C9H20 35 C5H12 3 C10H22 75 C6H14 5 C20H42 366,319 C7H16 9 C40H82 62,481,801,147,341 C8H18 18

Constitutional isomers usually have different physical properties Hexane Isomers (C6H14) Formula M.P. (oC) B.P. Density (g/mL) Refractive Index -95 68.7 0.6594 1.3748 -153.7 60.3 0.6532 1.3714 -118 63.3 0.6643 1.3765 -128.8 58 0.6616 1.3750 -98 49.7 0.6492 1.3688

IUPAC Nomenclature of Alkanes, Alkyl Halides, & Alcohols One of the most commonly used nomenclature systems that we use today is based on the system and rules developed by the International Union of Pure and Applied Chemistry (IUPAC) Fundamental Principle: Each different compound shall have a unique name

Although the IUPAC naming system is now widely accepted among chemists, common names (trivial names) of some compounds are still widely used by chemists and in commerce. Thus, learning some of the common names of frequently used chemicals and compounds is still important

The ending for all the names of alkanes is –ane The names of most alkanes stem from Greek and Latin one meth- two eth- three prop- four but- five pent-

Unbranched alkanes Name Structure Methane CH4 Hexane CH3(CH2)4CH3 Heptane CH3(CH2)5CH3 Propane CH3CH2CH3 Octane CH3(CH2)6CH3 Butane CH3CH2CH2CH3 Nonane CH3(CH2)7CH3 Pentane CH3(CH2)3CH3 Decane CH3(CH2)8CH3

3A. Nomenclature of Unbranched Alkyl Groups Removal of one hydrogen atom from an alkane

Alkyl group (Cont’d) For an unbranched alkane, the hydrogen atom that is removed is a terminal hydrogen atom

3B. Nomenclature of Branched-Chain Alkanes Rule Use the longest continuous carbon chain as parent name NOT

Rule (Cont’d) Use the lowest number of the substituent Use the number obtained by Rule 2 to designate the location of the substituent NOT

Rule (Cont’d) For two or more substituents, use the lowest possible individual numbers of the parent chain The substitutents should be listed alphabetically. In deciding alphabetical order, disregard multiplying prefix, such as “di”, “tri” etc.

Rule (Cont’d) NOT NOT

Rule (Cont’d) When two substituents are present on the same carbon, use that number twice

Rule (Cont’d) For identical substituents, use prefixes di-, tri-, tetra- and so on NOT NOT

Rule (Cont’d) When two chains of equal length compete for selection as parent chain, choose the chain with the greater number of substituents NOT

Rule (Cont’d) When branching first occurs at an equal distance from either end of the longest chain, choose the name that gives the lower number at the first point of difference NOT

Example 1 Find the longest chain as parent

Example 1 (Cont’d) Use the lowest numbering for substituents Substituents: two methyl groups dimethyl

Example 1 (Cont’d) Complete name

Example 2

Example 2 (Cont’d) Find the longest chain as parent

Example 2 (Cont’d) Find the longest chain as parent ⇒ Nonane as parent

Example 2 (Cont’d) Use the lowest numbering for substituents

Example 2 (Cont’d) Substituents 3,7-dimethyl 4-ethyl

Example 2 (Cont’d) Substituents in alphabetical order Ethyl before dimethyl (recall Rule 4 – disregard “di”) Complete name

3C. Nomenclature of Branched Alkyl Groups For alkanes with more than two carbon atoms, more than one derived alkyl group is possible Three-carbon groups

Four-carbon groups

A neopentyl group

Example 1

Find the longest chain as parent Example 1 (Cont’d) Find the longest chain as parent 6-carbon chain 7-carbon chain 8-carbon chain 9-carbon chain

Example 1 (Cont’d) Find the longest chain as parent ⇒ Nonane as parent

Use the lowest numbering for substituents Example 1 (Cont’d) Use the lowest numbering for substituents 5,6 4,5 (lower numbering) ⇒ Use 4,5

Example 1 (Cont’d) Substituents Isopropyl tert-butyl ⇒ 4-isopropyl and 5-tert-butyl

Example 1 (Cont’d) Alphabetical order of substituents tert-butyl before isopropyl Complete name

Example 2

Find the longest chain as parent Example 2 (Cont’d) Find the longest chain as parent 8-carbon chain 9-carbon chain ⇒ Octane as parent 10-carbon chain

Example 2 (Cont’d)

Use the lowest numbering for substituents Example 2 (Cont’d) Use the lowest numbering for substituents 5,6 5,6 ⇒ Determined using the next Rules

Example 2 (Cont’d) Substituents sec-butyl Neopentyl But is it 5-sec-butyl and 6-neopentyl or 5-neopentyl and 6-sec-butyl ?

Example 2 (Cont’d) Since sec-butyl takes precedence over neopentyl 5-sec-butyl and 6-neopentyl Complete name

3D. Classification of Hydrogen Atoms 1o hydrogen atoms 3o hydrogen atoms 2o hydrogen atoms

3E. Nomenclature of Alkyl Halides Rules Halogens are treated as substituents (as prefix) F: fluoro Br: bromo Cl: chloro I: iodo Similar rules as alkyl substituents

Examples

3F. Nomenclature of Alcohols IUPAC substitutive nomenclature: a name may have as many as four features Locants, prefixes, parent compound, and suffixes

Rules Select the longest continuous carbon chain to which the hydroxyl is directly attached. Change the name of the alkane corresponding to this chain by dropping the final –e and adding the suffix –ol Number the longest continuous carbon chain so as to give the carbon atom bearing the hydroxyl group the lower number. Indicate the position of the hydroxyl group by using this number as a locant

Examples

Example 4

Find the longest chain as parent Example 4 (Cont’d) Find the longest chain as parent Longest chain but does not contain the OH group 7-carbon chain containing the OH group ⇒ Heptane as parent

Use the lowest numbering for the carbon bearing the OH group Example 4 (Cont’d) Use the lowest numbering for the carbon bearing the OH group 2,3 (lower numbering) 5,6 ⇒ Use 2,3

Example 4 (Cont’d) Parent and suffix 2-Heptanol Substituents Propyl Complete name 3-Propyl-2-heptanol

How to Name Cycloalkanes 4A. Monocyclic Compounds Cycloalkanes with only one ring Attach the prefix cyclo-

Substituted cycloalkanes

Example 1

Example 2 (lowest numbers of substituents are 1,2,4 not 1,3,4)

Example 3 (the carbon bearing the OH should have the lowest numbering, even though 1,2,4 is lower than 1,3,4)

Cycloalkylalkanes When a single ring system is attached to a single chain with a greater number of carbon atoms When more than one ring system is attached to a single chain

4B. Bicyclic Compounds Bicycloalkanes Alkanes containing two fused or bridged rings Total # of carbons = 7 Bicycloheptane Bridgehead

Example (Cont’d) Between the two bridgeheads Two-carbon bridge on the left Two-carbon bridge on the right One-carbon bridge in the middle Complete name Bicyclo[2.2.1]heptane

Other examples

Nomenclature of Alkenes & Cycloalkenes Rule Select the longest chain that contains C=C as the parent name and change the name ending of the alkane of identical length from –ane to –ene

Rule Number the chain so as to include both carbon atoms of C=C, and begin numbering at the end of the chain nearer C=C. Assign the location of C=C by using the number of the first atom of C=C as the prefix. The locant for the alkene suffix may precede the parent name or be placed immediately before the suffix

Examples

Rule Indicate the locations of the substituent groups by the numbers of the carbon atoms to which they are attached Examples

Examples (Cont’d)

Rule Number substituted cycloalkenes in the way that gives the carbon atoms of C=C the 1 and 2 positions and that also gives the substituent groups the lower numbers at the first point of difference

Example

Rule Name compounds containing a C=C and an alcohol group as alkenols (or cycloalkenols) and give the alcohol carbon the lower number Examples

Examples (Cont’d)

Rule Vinyl group & allyl group

Rule Cis vs. Trans Cis: two identical or substantial groups on the same side of C=C Trans: two identical or substantial groups on the opposite side of C=C

Example

Example (Cont’d)

Example (Cont’d) Complete name

Nomenclature of Alkynes Alkynes are named in much the same way as alkenes, but ending name with –yne instead of –ene Examples

Examples (Cont’d)

OH group has priority over C≡C

Physical Properties of Alkanes & Cycloalkanes Boiling points & melting points

C6H14 Isomer Boiling Point (oC) 68.7 63.3 60.3 58 49.7

Physical Constants of Cycloalkanes # of C Atoms Name bp (oC) mp (oC) Density Refractive Index 3 Cyclopropane -33 -126.6 - 4 Cyclobutane 13 -90 1.4260 5 Cyclopentane 49 -94 0.751 1.4064 6 Cyclohexane 81 6.5 0.779 1.4266 7 Cycloheptane 118.5 -12 0.811 1.4449 8 Cyclooctane 149 13.5 0.834

Sigma Bonds & Bond Rotation Two groups bonded by a single bond can undergo rotation about that bond with respect to each other Conformations – temporary molecular shapes result from a rotation about a single bond Conformer – each possible structure of conformation Conformational analysis – analysis of energy changes occur as a molecule undergoes rotations about single bonds

8A. Newman Projections

8B. How to Do a Conformational Analysis

60o 0o 180o

f = 0o

Conformational Analysis of Butane

The Relative Stabilities of Cycloalkanes: Ring Strain Cycloalkanes do not have the same relative stability due to ring strain Ring strain comprises: Angle strain – result of deviation from ideal bond angles caused by inherent structural constraints Torsional strain – result of dispersion forces that cannot be relieved due to restricted conformational mobility

10A. Cyclopropane sp3 hybridized carbon (normal tetrahedral bond angle is 109.5o) q Internal bond angle (q) ~60o (~49.5o deviated from the ideal tetrahedral angle)

10B. Cyclobutane q Internal bond angle (q) ~88o (~21o deviated from the normal 109.5o tetrahedral angle)

Cyclobutane ring is not planar but is slightly folded. If cyclobutane ring were planar, the angle strain would be somewhat less (the internal angles would be 90o instead of 88o), but torsional strain would be considerably larger because all eight C–H bonds would be eclipsed

10C. Cyclopentane If cyclopentane were planar, q ~108o, very close to the normal tetrahedral angle of 109.5o However, planarity would introduce considerable torsional strain (i.e. 10 C–H bonds eclipsed) Therefore cyclopentane has a slightly bent conformation

Conformations of Cyclohexane: The Chair & the Boat

The boat conformer of cyclohexane is less stable (higher energy) than the chair form due to Eclipsed conformation 1,4-flagpole interactions

The twist boat conformation has a lower energy than the pure boat conformation, but is not as stable as the chair conformation

Energy diagram

Equatorial hydrogen atoms in chair form Substituted Cyclohexanes: Axial & Equatorial Hydrogen Atoms Equatorial hydrogen atoms in chair form Axial hydrogen atoms in chair form

Substituted cyclohexane Two different chair forms

The chair conformation with axial G is less stable due to 1,3-diaxial interaction The larger the G group, the more severe the 1,3-diaxial interaction and shifting the equilibrium from the axial-G chair form to the equatorial-G chair form

At 25oC G % of Equatorial % of Axial F 60 40 CH3 95 5 iPr 97 3 tBu > 99.99 < 0.01

Disubstituted Cycloalkanes Cis-Trans Isomerism

Trans-1,4-Disubstituted Cyclohexanes 13A.Cis-Trans Isomerism & Conformation Structures of Cyclohexanes Trans-1,4-Disubstituted Cyclohexanes

Upper bond Lower bond Upper-lower bonds means the groups are trans

Cis-1,4-Disubstituted Cyclohexanes

Cis-1-tert-Butyl-4-methylcyclohexane

Trans-1,3-Disubstituted Cyclohexanes

Trans-1-tert-Butyl-3-methylcyclohexane

Cis-1,3-Disubstituted Cyclohexanes

Trans-1,2-Disubstituted Cyclohexanes

Cis-1,2-Disubstituted Cyclohexane

Bicyclic & Polycyclic Alkanes

C60 (Buckminsterfullerene)

Synthesis of Alkanes and Cycloalkanes 16A.Hydrogenation of Alkenes & Alkynes

Examples

Index of hydrogen deficiency (IHD) How to Gain Structural Information from Molecular Formulas & Index of Hydrogen Deficiency Index of hydrogen deficiency (IHD) The difference in the number of pairs of hydrogen atoms between the compound under study and an acyclic alkane having the same number of carbons Also known as “degree of unsaturation” or “double-bond equivalence” (DBE)

Index of hydrogen deficiency (Cont’d) Saturated acyclic alkanes: CnH2n+2 Each double bond on ring: 2 hydrogens less Each double bond on ring provides one unit of hydrogen deficiency

e.g. Hexane: C6H14 C6H12 C6H14 – C6H12 Index of hydrogen deficiency (IHD) = H2 = one pair of H2 = 1

Examples IHD = 2 IHD = 3 IHD = 2 IHD = 4

16A.Compounds Containing Halogen, Oxygen, or Nitrogen For compounds containing Halogen – count halogen atoms as though they were hydrogen atoms Oxygen – ignore oxygen atoms and calculate IHD from the remainder of the formula Nitrogen – subtract one hydrogen for each nitrogen atom and ignore nitrogen atoms

Example 1: IHD of C4H6Cl2 Count Cl as H C4H6Cl2 ⇒ C4H8 A C4 acyclic alkane: C4H2(4)+2 = C4H10 C4H10 – C4H8 H2 IHD of C4H6Cl2 = one pair of H2 = 1 Possible structures

Example 2: IHD of C5H8O Ignore oxygen C5H8O ⇒ C5H8 A C5 acyclic alkane: C5H2(5)+2 = C5H12 C5H12 – C5H8 H4 IHD of C4H6Cl2 = two pair of H2 = 2 Possible structures

Example 3: IHD of C5H7N Subtract 1 H for each N C5H7N ⇒ C5H6 A C5 acyclic alkane: C5H2(5)+2 = C5H12 C5H12 – C5H6 H6 IHD of C4H6Cl2 = three pair of H2 = 3 Possible structures

 END OF CHAPTER 4 